Method of making a cross-rifled vapor generating tube

ABSTRACT

A cross-rifled vapor generating tube is formed by rotating a smooth tube within a die at a first spiral lead angle to form a number of spiral grooves on the inside surface thereof and subsequently cold-drawing a number of grooves having a second spiral lead angle to thereby form a number of regularly spaced projections on the inside surface of the tube. The lead angles of the first and second spirally formed grooves are reversed with respect to one another and the sum of their angles equals 20*80*. An improved generating tube is formed when the first and second spiral lead angles do not exceed 43*. In a modified embodiment the second spiral lead angle is zero.

Nakamura et al.

[ Aug. 20,1974

METHOD OF MAKING A CROSS-RIFLED VAPOR GENERATING TUBE Inventors: Hisashi Nakamura, lbaragi;

Masatoshi Tanaka, Nishinomiya, both of Japan Assignee:

Osaka, Japan Filed: July 29, 1971 Appl. N0.: 167,442

Related US. Application Data Division of Ser. No. 51,434,-Ju1y l, 1970, Pat. No. 3,734,140.

72/703; 29/157.3 AH, 157.3 A; 138/177 References Cited UNITED STATES PATENTS 8/1955 Grob 29/1573 A Sumitomo Metal Industries Limited,

3,289,451 12/1966 Koch 72/283 3,292,408 12/1966 Hill 3,504,513 4/1970 Black 72/703 X Primary ExaminerChar1es W. Lanham Assistant ExaminerRobert M. Rogers ABSTRACT A cross-rifled vapor generating tube is formed by rotating a smooth tube within a die at a first spiral lead angle to form a number of spiral grooves on the inside surface thereof and subsequently cold-drawing a number of grooves having a second spiral lead angle to thereby form a number of regularly spaced projections on the inside surface of the tube. The lead angles of the first'and second spirally formed grooves are reversedwith respect to one another and the sum of their angles equals 20-80. An improved generating tube is formed when the first and second spiral lead angles do not exceed 43. In a modified embodiment the second spiral lead angle is zero.

4 Claims, 10 Drawing Figures TEMPERATUYZE Pmm'ggauczolm 3,830,087 SiiiET a 600- L M max SMOOTH TUBE HEM FLUX 4on0? 1 crzoss RtFLED TUBE HENY FLUX SOHO? kcai/rnkhr moss RTFLEB TUBE AT FLUX 40 x10 kcd/rrP-hr SMOOTH TUBE HEAT FLUX M04 kco /m hr I.

I SMOOTH TUBE 1 4 0" 11 HEAT ELux y ,n I

\oxm kcoflm hr A4 craoss RlFLED TUBE 1on0 kccfl/m' -hr o s 300 t 2555 R\FLED TUBE SQ; 33 TUBE T FLUX A B \O X \0 4 kcowm -hr QQHO C0\ hr QUMATY H Y i kcal/ kg.

PATENTEUAUEZOW 3,830,087

SHEET s 0? 4 FIG. 9

a \Nxi- 1 i i :5 B 2 \16 13 This is a division of application Ser. N. 51,434, filed July 1, 1970, now US. Pat. No. 3,734,140.

This invention relates to improvements in the manufacture of vapor generating tubes to be operated mostly under a pressure below the critical pressure and to be subjected to high heat flux, and more particularly to a method for making a cross-rifled vapor generating tubeprovided uniformly with many projections of a rhombic or parallelogramic shape made on the inside wall surface by cross-rifling the inside surface.

More particularly, the above mentioned uniform projections are regularly set by cross-rifiing the inside surface of a tube to be used mostly as a wall tube for a high temperature-high pressure boiler using a fossil fuel (such as coal, heavy oil or natural gas), so that there is produced a remarkable improvement in the critical heat flux so as to be higher than in a smooth tube by promoting the maintenance of nucleate boiling of the fluid passing through the tube.

Further, not only in two-phase flow but also in singlephase flow, the heat transfer is remarkably promoted,

Therefore, when the cross-rifled tubes of this invention are used for general heat-exchangers having no boiling or vapor generating tubes operated under supercritical pressure, a remarkable improvement is obtained.

The safety of the operation of vapor generating tubes of a boiler depends on the wall temperature of the tube. That is to say, it is related so closely with the coefficient of heat transfer on the inside surface of the tube, the temperature of the operating liquid (such as the mixture of steam and water) and the heat flux that it is necessary to always pay careful attention to the wall surface exposed to strong flame radiation during its operation. Today, due to the increase of the adoption of an oil burning system and the general use of a oncethrough boiler, there has increased the possibility of the local presence of a condition exceeding in some cases the critical heat flux causing physical burn-out (breakdown) of the vapor generating tube. As a countermeasure for increasing the critical heat flux, a tube called a ribbed tube made by forming spiral lands on.

the inside surface of a vapor generating tube is sugby the transition from nucleate boiling to film boiling but the latter is caused by the annular flow pattern of the gas-liquid mixture. Thus two kinds of burn-out of the vapor generating tube are caused by quite different mechanisms. Therefore, in order to make the critical heat flux in the vapor generating tube definite, it is an essential condition to measure the critical heat flux by heat transmission tests and to watch the flow pattern by flow tests.

In the drawings:

FIG. 1 is a sectional view of an embodiment of a cross-rifled vapor generating tube made according to the method of the present invention;

FIG. 2 is a sectional view showing another embodiment of the tube shown in Flg. 1;

FIG. 3 is a graph showing a comparison of inside surface temperatures of a smooth tube and a cross-rifled tube made in accordance with the method of the present invention in the case of a pressure of 210 atmo (Kg/cm and a mass velocity of 700 to 710 I(g./m sec.;

FIG. 4 is a graph showing a comparison of the maximum values of the inside surface temperatures to the heat fluxes of a smooth tube and a cross-rifled tube made in accordance with the method of the present invention in the case of a pressure of 210 atmo (Kg/cm FIG. 5 is a schematic view showing an apparatus for making a cross-rifled tube of the present invention;

FIG. 6 is a sectional view of apparatus in the first step of the method;

FIG. 7 is a sectional view of a tube during the first step of the method;

FIG. 8 is a sectional view of a bearing part of a mandrel;

FIG. 9 is a sectional view of apparatus in the second step for making the cross-rifled tube shown in FIG. 1.

FIG. 10 is a sectional view of apparatus in the second step for making the cross-rifled tube shown in FIG. 2.

The cross-rifled tube according to the present inven- Examples of the chemical compositions mechanical 5 properties and dimensions of the test tubes are shown in the following Tables 1 and 2.

TABLE I Chemical compositions and mechanical properties of cross-rifled tubes of the present invention Chemical Compositions (in be weight) C Si P S Test tubes Mn Ni Cr Mo gested, for example, in US. Pat. No. 3,088,494 (or Ca- Test Mech cal ro enies nadian Pat. NO. 684 tubes Tensrle strength(1n Kg/mm) Yield pomt (1n Kg/mm An object of the present invention is to provide an g 25 improved method of making a cross-rifled generating tube of the type specified herein.

Generally, the burn-out phenomenon of a vapor generating tube subjected to high heat flux and used below h 'l th htt ns' tof twof ctors Mham?l ewes t e crttlca pressure 15 oug o co 1s a Tesnubes Elongation (m of fast burn-out (also called heat flux burn-out) 1n a low quality region and slow burn-out (also called enthalpy Q gg-g burn-out) in a high quality region. The former is caused Table 2 Dimensional data of cross-rifled test tubes Test tubes A-l A-2 B-l B2 Outside diameter D (in mm) 20.27 20.16 20.99 20.06 Minor inside diameter d, (in mm) 9.66 9.54 13.27 13.18 Height h (in mm) of the projection 0.55 0.64 0.47 0.52 Number of spiral 12 12 12 12 (number per cross-section) Width h (in mm) of the projection 3.16 3.16 3.75 3.75 Lend angle u 3 2220 2220 2148 2148 Lend angle [1 1845 1845 1837 1837 Lend angle u 1 [3 4105 4105 4025 4025 Pitch p (in mm)[=lead right 732 7.32 11.20 11.20 (in mm) X number of spisals] left 6.88 6.88 8.65 8.65

P/h right 13.30 11.42 23.80 21.60 Ratio left 12.50 10. 78 18.40 16.65 condition h/d 0.057 0.067 0.036 0.040

b/p right 0.432 0.432 0.335 0.335

left 0.459 0.459 0.433

and the crossrifled tube of the present invention in the case ofa pressure of 210 atmo (Kg/cm and a mass velocity. of 700 to 710 kg/m sec. are shown for bulk average specific enthalpy with the heat flux as a parameter in FIG. 3. 1n the same graph, the heat flux q is taken in the range of X 10 to 50 X 10* kcal/m hr., the test data of the inside surface of the smooth tube are shown with solid lines and those of the cross-rifled tube according to the present invention are shown with dotted lines to compare the relative effects and a quality scale is also given on the abscissa for reference. As is obvious from this graph, in both a single-phase region and a two-phase region, in the case of the cross-rifled tube, the inside surface temperature is much lower than in the case of the smooth tube and the heat transfer is promoted. That is to say, even in the region in which the wall temperature rises quickly in the case of the smooth tube, the nucleate boiling is kept up to the high quality region in the caseof the cross-rifled tube. Therefore, the wall temperature of the cross-rifled tube is much lower than of the smooth tube.

Further, as can be seen from FIG. 3, the quick rise of the wall temperature in the case of the smooth tube begins below a quality of 50 percent. It is judged from these test results that the burn-out occurs substantially in the state of bubble flow and is so-called fast burn-out by the transition from nucleate boiling to film boiling.

The maximum value (Tw max.) of the wall temperature vs. the heat flux at that time as shown with the mass velocity as a parameter is as in FIG. 4. In this graph, the smooth tube and the cross-rifled tube of the present invention are compared with each other for the three conditions of mass velocities of 900, 700 and 400 kg/m sec. In the case of the cross-rifled tube of the present invention, the above mentioned maximum value (Tw max.) of the wall temperature is much lower than in the case of the smooth tube and, in this respect, too, the effect of the cross-rifled tube is shown to be remarkable.

It is shown that the cross-rifled tube of the present invention can well endure physical burn-out at a heat flux of 60 X 10 kcal/m hr. even under such severe condition as a mass velocity of 400 kg/m sec. As the maximum local heat flux of an oil burning boiler is 50 to 60 X 10 kcal/m hr., if such cross-rifled tube is used in high heat flux parts of an ordinary boiler using a fossil fuel, there is no danger of a physical burn-out and it contributes much to the design of a subcritical pressure boiler.

Further, the superiority of this cross-rifled tube can be proved even from the results of the two-phase airwater flow test at normal temperature and pressure. That is to say, according to the flow test, in the crossrifled tube, in the case of bubble flow, bubbles are more likely to concentrate in the center part of the tube than in the smooth tube and the ribbed tube of the prior art and, in the case of annular flow, the water film thickness becomes larger. Therefore, it is thought that the cross-rifled tube of this invention is superior to tubes of any other type and shape in both fast burn-out and slow burn-out and enables an increase in the critical heatv flux. Further, what is to be specifically noted is that the pressure drop in the cross-rifled tube in a single-phase flow and two-phase flow is small. This is a remarkable superiority to the ribbed tube having a large lead angle.

The cross-rifled of the present invention is made by using a cold-drawing process with a die and plug. First of all, in the first step, a plug on which a plurality of spiral grooves are made in advance is inserted into a tube and a plurality of spiral lands are formed on the inside surface of a smooth tube which is a mother tube by the free rotation of this plug. Then in the second step, another plug on which a plurality of spiral grooves with the lead angle reversed are made or on which straight grooves are made is inserted into the above mentioned tube in which the spiral lands are made and a part of the spiral lands made in that tube are plastically pressed down by the free rotation or straight drawing of this plug so that many projections of a rhombic or any shape may be uniformly and discontinuously made on the inside surface of the tube.

FIG. 5 is a side view showing a main part of a chain type colddrawing bench to be used in this invention. A mother tube 3 is continuously drawn by a working part 2 fixed to a bed 1 of the bench and a said carriage 4 towed by an endless chain 6. The top end of said mother tube 3 is pressed before the drawing and is inserted in the carriage 4. A pawl 5 of carriage 4 is suspended on the driving endless chain 6 so that carriage 4 may be towed.

As shown in FIG. 6, in the working part 2, a die holder 7 is fixed to the bed 1 of the bench body, and contains a die 8 and is provided with a hole 9 of a diameter a little larger than the inside diameter of the die in the center part of the die hole. A plug 11 borne by a mandrel may be inserted into mother tube 3 and tube 3 is drawn in this state. The plug 11 is provided with a plurality of spiral lands of fixed dimensions having a fixed lead angle as described above. When tube 3 is drawn between plug 11 and die 8, spiral lands [3 are plastically pressed and molded on the inside surface of the tube with spiral grooves made on the outside surface of the plug 11. For example, as shown in FIG. 7, tube 3 is made to complete the first step. In such a case, the bearing part of the mandrel 10 supporting plug 11 is provided with such rotating means as a thrust bearing 14, for example, as shown in FIG. 8 so that plug 11 may rotate smoothly and freely. Further, in the rear of bed 1 on the mandrel stopping side is set a known plug position adjusting device 15 as shown in FIG. 5.

Next, as shown in FIG. 9, tube 3 is used as a mother tube in the second step. A plug 17 is provided with spiral grooves having a lead angle in a direction reverse to that of the plug used in the first step and is supported by mandrel 10 and a rotary drawing is applied again to tube 3. Thus, spiral lands l3 molded by the first step are partly plastically deformed or pressed down by spiral lands 16 made on the outer peripheral surface of plug 17 and many regularly spirally arranged projections 18 of any rhombic or parallelogramic shape having both lead angles a and B are continuously formed on the inside wall of the tube. Even in this second step the mandrel bearing part supporting plug 17 is made of such a smoothly rotatable structure as is shown in FIG. 8 exactly the same as in the first step.

As mentioned above, many projections are set on the inside surface of the tube. The shape of the projection is different depending on the shape of the plug to be used or the combination of such dimensional data as the percentage of the area reduction. FIG. 10 shows such modification wherein many parallelogram shaped projections are set by making a straight drawing by using a plug 19 provided with grooves of a lead angle ,8 0, that is, straight grooves parallel with the axial direction of the tube in the second step.

From the results of broad tests, it has been found that the shape and arrangement of the projections of the cross-rifled tube of this invention are very advantageous to the heat transfer effect. As a result, it has been found that, in order that the cross-rifled tube of the present invention may maintain nucleate boiling from a low quality region to a high quality region for such given conditions as pressure, heat flux and mass velocity, it should have projections (lands satisfying the following rations and arrangement conditions;

P/h 5 to 40, h/d 0.005 to 0.08.

b/p 0.2 to 0.8 and a B 20 to 80 one cross-section, 11 represents a height of the projection, 11, represents a minor inside diameter of the tube and 11 represents a width of the projection as projected in the radial direction of the tube. Further, the shape I 5 P/ll s m 25.

h/lI' 0.0l to 0.07

[J/P 0.3 to 0.6 and a +3 30 to 75 In the above mentioned limitations of the respective magnitudes. the numbers of spirals counted in one cross-section and convenient to provide in the manufacture are 6, l2, l8 and 24 spirals. However, in the 5 tube of the present invention, 12 and I8 spirals are used. In the above mentioned Table 2, the respective ratio conditions of four kinds of test tubes are shown. As shown in the table, the respective test tubes are satisfactory within the range of all the ratio and arrange- 0 ment conditions. No great difference is recognized at all in the test results of these tubes. Among the above mentioned conditions, only the last mentioned arrangement condition of a B 20 to 80is a condition determined by the cold-drawing by the free rotation of the plug, that is, the limitation in the manufacture. The setting of the spiral lands on the inside surface of the tube by the free rotation of the plug is determined by the frictions between the plug and die and the tube and the maximum limit of each of a and B is 43.

The shape and arrangement of the projections in the cross-rifled tube bringing about the maximum critical heat flux are different depending on the pressure to be used. However, it has been confirmed that, in the above mentioned embodiment of a sub-critical pressure of 210 atmo (Kg/cm in the case of a mass velocity of 700 Kg/m sec. and a heat flux of 60 X 10 kcal/m hr., nucleate boiling is maintained until a quality of 70%. Further, at a mass velocity of 400 kg/m sec., it is possible to keep the allowable temperature of an ordinary boiler tube below 500C. under such severe condition as heat flux of 60 X 10 kcal/m hr. By using a crossrifled tube of the present invention, almost all the problems in the design of the boiler tube are obviated.

What is claimed is: 1. A method of making a cross-rifled vapor generating tube comprising the following steps:

forming a number of spiral lands on the inside surface of a smooth tube by rotation of a spirally grooved plug within said tube as the latter is drawn through a die, said grooves of said plug having a first spiral lead angle a; deforming said lands by repeating said operation with a second grooved plug having a second spiral lead angle B a plus B equalling 20 80;

and thus forming a number of regularly spaced projections on the inside surface of said tube. 2. A method as in claim 1 wherein a plus B equals 3075.

3. A method as in claim 2 wherein a plus B does not exceed 43.

4. A method as in claim 1 wherein B equals zero. 

1. A method of making a cross-rifled vapor generating tube comprising the following steps: forming a number of spiral lands on the inside surface of a smooth tube by rotation of a spirally grooved plug within said tube as the latter is drawn through a die, said grooves of said plug having a first spiral lead angle Alpha ; deforming said lands by repeating said operation with a second grooved plug having a second spiral lead angle Beta Alpha plus Beta equalling 20* - 80*; and thus forming a number of regularly spaced projections on the inside surface of said tube.
 2. A method as in claim 1 wherein Alpha plus Beta equals 30*-75*.
 3. A method as in claim 2 wherein Alpha plus Beta does not exceed 43*.
 4. A method as in claim 1 wherein Beta equals zero. 